While parthenogenesis, asexual reproduction in which embryos develop without fertilization, is the norm for some vertebrate species, researchers have begun to find evidence of a strange phenomenon called facultative parthenogenesis, in which females of species that usually reproduce sexually can deliver offspring without mating, Nature reported Wednesday.

Warren Booth, a molecular ecologist at the University of Tulsa in Oklahoma, reported pregnant copperheads and cottonmouths, captured in the wild, that later gave birth in a lab to litters, causing Booth to suspect they reproduced without male input.

Booth examined genetic markers in the mothers and offspring to determine if the young snakes were the result of facultative parthenogenesis.

"When I got the results of the DNA sequencer, I was floored," he said, noting the results indicated the chance of a male contribution was "infinitesimally small."

While researchers have always considered facultative parthenogenesis in the wild a possibility, Booth said he and his colleagues were "stunned" at finding the evidence.

It is not clear why asexual reproduction would evolve in species that are normally sexual, the researchers said.

Isolation from males is not a factor, Booth said, since the female snakes were collected from habitats with plenty of males present.

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